Engine Backfires

WARNING: This page is about a different car, the 1997 Jaguar XJR. However, it is still accessible from the selected car via links, so may be relevant.

Air leak into/from exhaust system.
Air leak past valve guides or valves.
Incorrect ignition timing.
Incorrect valve timing.
Intake manifold air leak.
Lean air/fuel mixture.
Valves sticking open.